This invention relates to methods of and circuits for suppressing radar output due to clutter and noise while retaining desired output due to targets that are moving with velocities different than those of clutter. The invention is applicable to radars that operate on the doppler principle of detection and therefore it is applicable to cw doppler, pulse doppler and MTI (moving target indication) radars.
The principles of the three aforementioned types of radars are given in Chaps. 3 and 4 of the book Introduction of Radar Systems, McGraw-Hill Book Company, 1980, by M. I. Skolnik. Throughout this document the term "doppler radar" refers to either MTI, cw or pulse doppler radar.
This disclosure relates to the detection of moving targets with doppler radar for which the target echoes have relatively high doppler frequencies, and the rejection of land and sea clutter and noise. Radar clutter signals are unwanted signals caused by radar echo (see, e.g., Skolnik, p. 470), and clutter signal magnitude therefore depends on radar range and direction. Land and sea clutter, being from stationary and slowly moving objects, have relatively small doppler frequencies including frequencies at and near zero. Noise signals, on the other hand, are unwanted signals caused by random fluctuations having wide doppler frequency spectra extending from zero to an upper limit controlled by receiver bandwidth (Skolnik, pp. 23-29). Unlike clutter echo, noise at the radar output occurs at all radar ranges.
Based on the relative strengths of the doppler frequency components contained in the radar echo, doppler processors use filters to suppress signals from clutter and to retain signals from moving targets. These filters can be implemented in analog or digital hardware, or in software. Ideally the output of a doppler radar due to clutter is zero. However, a radar has frequency and amplitude instabilities that cause its doppler processed output to fluctuate, even though the echo is from a stationary object. Another cause for fluctuations in echo strength, which provides another source of non-zero doppler frequencies, is beam movement across a reflecting object due to antenna scanning. Therefore, radar output after doppler processing caused by stationary objects (clutter) is oftentimes much stronger than the output caused by some (the weaker) moving targets of interest.
The usual technique for rejecting clutter and noise is to employ an amplitude threshold level below which signals are rejected. Unfortunately, the threshold rejects the weaker targets and passes the stronger clutter and therefore amplitude thresholding is satisfactory only if, at the output of the doppler processor, the important target signals are stronger than most of the clutter and noise.
Modern automatic detection radars use a thresholding circuit called CFAR (constant false alarm ratio). The CFAR operates by adaptively following interference (whether from receiver or external noise or from clutter) and adjusting the threshold level so as to automatically reject the interference in each range cell (Skolnik, pp. 392-395). CFAR performance is effective against homogeneous clutter and noise. However, CFARs are inefficient suppressors of typical surface (ground and/or sea) clutter, which is not homogeneous and often consists of isolated and very strong clutter areas interspersed with regions of negligible clutter.
The most commonly used CFAR is called a range CFAR. A range CFAR sets its threshold in each range cell based on sampling, during each range sweep, the strengths of signals in its neighboring cells. Next, it combines and obtains a statistic (usually an average) of the sampled signals, and it sets a threshold for said each range cell, Then for each range cell, a radar signal is outputted if it exceeds its threshold. Thus, a detection is declared at a given range cell if it contains a signal that exceeds its threshold based on its aggregate of signals from nearby cells. Another well known type of CFAR is called a doppler CFAR. It samples doppler cells (filters), which are numerous in some pulse doppler radars (see, e.g., Patton and Ringel, U.S. Pat. No. 3,701,149). Then, based on a sampling of doppler cells, a threshold is set for each range cell.
Modern radars have a large number of range cells, and therefore CFAR processing is complex. In addition, a common weakness of CFARs is lack of robustness, i.e., inability to process effectively a wide variety of clutter types. Various trade-offs in CFAR designs can be made to accommodate problems of detection efficiency and false alarms that are created by both (1) a wide variety of clutter types and (2) the presence of multiple, closely spaced targets within nearby range cells (see, e.g., G. V. Morris, Airborne Pulse Doppler Radar, Artech House, 1988, Chapter 17). However, the changes in design required for solving these problems contribute to additional complexity; and thus contribute to further increases in size, weight and costs.
U.S. Pat. Nos. 4,459,592 and 4,684,950 teach ratio comparator methods and means for establishing an adaptive clutter threshold level to reject isolated clutter even though it is very strong. More specifically, a ratio comparator technique is taught that functions on the basis of the ratio of the amplitudes of two signals: one proportional to the radar received signal and which contains doppler frequencies at and near zero--and the other proportional to the output of the doppler processor.
U.S. Pat. No. 4,684,950 teaches additional clutter thresholding that cooperatively operates with the basic ratio comparator technique for improving clutter suppression if the suppression is impaired by a limited receiver amplitude dynamic range or by clutter fluctuations caused, e.g., by wind blown trees. The additional thresholding of U.S. Pat. No. 4,684,950 functions specifically on the amplitude of the radar received signal prior to doppler processing, in relation to at least one fixed amplitude level. On the other hand, the present disclosure teaches a noise thresholder employed after doppler processing in operative association with the above-mentioned ratio comparator technique. In this combination, the new invention suppresses both the clutter and noise at the radar output over the full range of clutter and noise mixes, from clutter with imperceptible noise to noise with imperceptible clutter.
U.S. Pat. Nos. 4,459,592 and 4,684,950 teach the use of a CFAR in combination with the ratio comparator technique to provide improved target detection and clutter suppression performance over that attainable with only a CFAR. As discussed later in this disclosure, the present invention has a number of advantages over the ratio comparator/CFAR combination previously taught, including less complexity and greater sensitivity for detecting targets in land and sea clutter.